Method of making an electric firing device



1960 e. H. SCHERRER 2,924,140 Y METHOD OF MAKING AN ELECTRIC FIRING DEVICE Filed Sept. 9. 1949 any! INVENTOR.

GEORGE H. SCHERRER ATTORNEY METHOD or MAKING AN ELECTRIC FIRING DEVICE George H. Scherrer, Port Ewen, N.Y., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application September 9, 1949, Serial No. 114,712

1 Claim. (Cl. 86-1) The present invention relates to electric firing devices and the process of making them, and more specifically.

to firing devices of the kind wherein the igniting means is a bridge" composed of electrically conductive particles, in relatively poor contact with one another, resulting in a high electrical resistance, well suited to becomeheated sufiiciently to ensure ignition by an electrical discharge of low energy.

Heretofore, difficulty has been experienced with bridges of this kind, due to inherent mechanical instability and lack of strength of the assembly of separate, particles. Attempts to avoid this defect usually involved strengthening the bridge by coating it with a thin film, such as lacquer, or mixing a binder with the particles. The coating of lacquer, however, prevented good contact with the ignition powder while the binder increased the compactness and mass of the bridge, with the result that much more electrical energy was required to heat the bridge sufficiently.

An object of the invention is to overcome these difiiculties, and to provide an electric firing device of the bridge type that is stable and highly sensitive.

Other objects and many of the attendant advantages of this invention will be appreciated readily as the same becomes understood by reference to the following detailed description, when considered in connection with the accompanying drawings, wherein:

Fig. 1 is a fragmentary partly sectional side elevation, on a greatly enlarged scale, of an igniter plug embodying the inventio'n; and

Fig. 2 is a section through the plug in the plane 2-2 of Fig. 1.

In making firing devices embodying the present invention, the first step is to provide igniter plugs. These are prepared by twisting together a pair of conductors, 1, 2 insulated with enamel, polyvinyl acetal, or other similar material that produces a thin flexible insulating layer on the conductor. These twisted pairs of co'nductors are then embedded in a suitable plastic by means of a mold that will yield plugs of the desired shape and size, usually small cylinders 3.

A shoulder is formed at 4 to receive a ferrule 5, which may be made of any desired material, such as metal, plastic, vulcanized fibre, and other substances, force-fitted into place. If the ferrule is non-metallic, it is preferred to provide additional security of attachment by a suitable glue or cement.

Before the ferrule is put in place, however, the end face 6 of the plug 3 is ground and polished to provide a smooth surface in which the exposed ends of conductors 1 and 2 lie flush.

From Fig. 2 it will be seen that the end faces of conductors 1 and 2 are closely adjacent each other in the surface 6, and are separated by the double thickness of their insulating layers, 7 and 8. Thus a spacing of l to 1.5 mils is provided between the conductors. This relatively small gap must next be bridged by a suitable high resistance bridge 9, Fig. 1, a p

According to the present invention, this may be accomplished in several ways. Graphite is the preferred resistance material. This is commercially available in colloidal suspension, in one make, under the trade name aquadag (aqueous deflocculated Acheson graphite) but it is not intended to limit the invention to this particular substance, as equivalents may be used.

The aquadag is applied to the near-junction of the conductors 1 and 2 in the form of a droplet of diluted aquadag solution of about 0.2% to 0.5% graphite content. Various ways of applying the solution are possible, as by a brush, a dropper, or actual dipping of the face of the plug into the solution. Drying may be speeded by use of heat or air circulation, but this is not necessary for satisfactory performance.

When dry, the resulting bridge should have a resistance,

of the order of 2000 to 5000 ohms, but these figures are merely suggestive, and may be raised or lowered if preferred. The cavity provided by the ferrule 5 serves to receive an ignition powder 10, which is to be ignited by the bridge and in turn serves to augment and transmit the ignition as desired.

Various ignition powders are available, but powdered lead styphnate is preferred, because it gives the greatest energy sensitivity. A measured volume of the powder is pressed into the ferrule, as indicated at 10. The plug, thus loaded, is then press-fitted into a metal shell which contains the required amount of loose explosive, said explosive becoming compressed by the plug as a result.

The above described assembly can be made to have extreme sensitivity to ignition by electrical current, and particularly torvery short current pulses, such as those produced by the discharge of. a capacitor. However, the

resistance of graphite bridges is not stable in this type of assembly and such instability adversely aifects their operability in series circuits.

Graphite particles applied as dilute aquadag form a loose and porous bridge on drying. For a given bridgeresistance, much more material is present than would be necessary if it were compacted. When the ignition charge 10 is pressed into the ferrule and against the bridge 9, the graphite becomes compressed and the totalresistance decreases. This in itself is not seriously objectionable, because it could be compensated by making the initial bridge resistance correspondingly too high.

However, further changes in resistance take place after the assembly is complete, the amount and nature of such changes depending upon the storage conditions, particularly the temperature, to which the devices are subjected. Presumably this is due to the fact that dimensional changes due to expansions and/ or contractions of the various igniter components will create pressures on the bridge and compact if further. Failure to load the igniter in a manner that will apply and maintain good contact between the bridge and the ignition powder results in poor ignition sensitivity.

As previously stated, attempts to avoid these defects by coating the bridge with a film of stifiening material, such as lacquer, for example, or mixing a binder with the particles, were not successful because they either prevented good electrical contact between the bridge and the conductors, or increased the compactness and mass of the bridge, thus increasing the electrical energy required to heat it sufiiciently to provide ignition.

The present invention stabilizes the electrical resistance of the bridge and maintains good electrical contact therewith, by mixing lacquer or the like, not with the bridge material, but with the ignition initiating powder instead. The mixture is then applied over the bridge to fill the ferrule and is permitted to dry in place. It is true that some of the lacquer binder then penetrates the graphite, to give mechanical support, but not to an extent sufiicient to harmfully increase the mass of the bridge.

n the contrary, in penetrating the graphite of the bridge the lacquer carries with it fine particles of the ignition powder in suspension, which it deposits at the boundary layer of said bridge, thus ensuring intimate mechanical contact between said graphite and the ignition powder, which in turn improves the ignition of the latter by the graphite. As the lacquer dries, the ferrule remains filled with powder mixed with a binder, consisting of the lacquer solids. This forms a firm adherent coating which will protect the bridge from base-charge pressure.

There are several acceptable ways of charging the ferrule with the lacquer-powder mixture. The ignition powder may be mixed with liquid lacquer to a suitable plastic consistency and placed in the ferrule by means of a spatula. For this procedure, the mixture should be fairly soft, so as to wet the bridge surface thoroughly. Care is necessary to exclude air bubbles, which would detract from the efficiency of this process.

A preferred procedure, however, is to place a thin primer coat over the bridge, in the form of a drop from a wire loop, and after allowing this to dry, to fill the remainder of the cavity in the ferrule by use of a spatula. This ensures that the powder is placed correctly over the bridge, and promotes rapid drying of the first increment of powder. An added advantage thus secured is that the plug, wire insulation, and bridge are not subjected to the solvent action of the lacquer vehicle for as long a period.

While various types of lacquer may be used, it is preferred to standardize on a single kind, for it is necessary to maintain the proportion of binder in the powder as nearly uniform as readily feasible, in order to attain both electrical-resistance stability and good firing sensitivity. Clear Egyptian lacquer having a 12% solids content is satisfactory. The solids may be 82% nitrocellulose and 18% gum elemi. One gram of this lacquer is mixed with two grams of lead styphnate until the mixture is smooth. A droplet of the mixture is placed over the bridge by the use of a small wire loop. As a guide, it may be stated that the powder when dried should weigh about 1.5 milligrams. After drying, the remainder of the ferrule cavity is filled with lead styphnate, wetted to suitable handling consistency with the same lacquer. When this too has dried, the plug assembly is press-fitted into a shell, resting firmly against the desired charge of powder. It will be noted that the lacquers that are suitable for the present invention are such as will burn readily "even in the absence of an oxygen atmosphere, and such substances, for lack of a betterterm, are herein called pyrotechnic materials.

Firing plugs made as above described have been stored for over a year at F. without significant change in electrical-resistance. After such storage they still fired satisfactorily, whereas under similar conditions a large percentage of firing plugs having dry-assembled ignition powder would become inacceptable for military use, because their resistances would have dropped excessively.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claim the invention may be practiced otherwise than as specifically described.

What is claimed is:

The method of constructing an electric firing device having a stabilized resistance and adapted to be heated by a low energy electrical discharge comprising, twisting a pair of insulated conductors together, embedding said conductors in plastic thereby providin a plug, forming a shoulder on said plug for receiving a ferrule, grinding and polishing an end face of said plug to provide a smooth surface in which the ends of said conductors lie flush, placing a ferrule on the plug so as to provide a cavity adjacent the end face of said plug, applying a coating of graphite in colloidal suspension over the ends of said conductors to provide a resistance bridge between said conductors, drying said coating, applying a coating of lacquer and ignition powder mixture over said bridge, and filling the cavity formed by said ferrule with lacquer and ignition powder mixture.

References Cited in the file of this patent UNITED STATES PATENTS 39,542 Beardslee Aug. 18, 1863 374,640 Sundstrom Dec. 13, 1887 1,614,210 Schmitt Jan. 11, 1927 2,186,427 Nash Jan. 9, 1940 2,205,081 Burrows et al. June 18, 1940 2,235,009 Campbell Mar. 18, 1941 FOREIGN PATENTS 578,300 Great Britain June 24, 1946 

